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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Closed: (HADOOP-200) The map task names are sent to the reduces
Date Mon, 05 Jun 2006 23:08:44 GMT
     [ http://issues.apache.org/jira/browse/HADOOP-200?page=all ]
     
Doug Cutting closed HADOOP-200:
-------------------------------


> The map task names are sent to the reduces
> ------------------------------------------
>
>          Key: HADOOP-200
>          URL: http://issues.apache.org/jira/browse/HADOOP-200
>      Project: Hadoop
>         Type: Bug

>   Components: mapred
>     Versions: 0.2
>     Reporter: Owen O'Malley
>     Assignee: Owen O'Malley
>      Fix For: 0.3.0
>  Attachments: map-id.patch
>
> As each reduce is created, it is given the entire set of potential map names. For my
large sort jobs with 64k maps, this means that each reduce task is given a two dimensional
array that is 5 tasks/map * 64k maps = 320k strings. Since the reduce task is passed from
the job tracker to the task tracker and down to the task runner, passing the entire list is
very expensive. I suspect that this is the cause of the slow downs that I see in the task
trackers heart beats when the reduce tasks are being launched.
> I propose that the ReduceTask be changed to just get the count of maps, with ids from
0 .. maps -1.
>   public ReduceTask(String jobFile, String taskId, int maps, int partition);
> Then we need to change the protocol for finding map outputs:
>   MapOutputLocation[] locateMapOutputs(String jobId, int[] mapIds, int partition);

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